Montenegro officially became the 29th member of the North Atlantic Treaty Organization on Monday during a ceremony in Washington at the US State Department's Treaty Room. The US was one of the organization's 12 founding members in 1949.

US President Donald Trump was not present. Thomas Shannon, a State Department undersecretary and department's third-highest official (photo left) met with Prime Minister Dusko Markovic and Montenegrin Foreign Minister Srdjan Darmanovic for the ceremony.

It was Markovic whom Trump pushed out of the way during a NATO photo call at a summit in Belgium.

"We are celebrating today the fact that it will never happen again that someone else decides instead of us and our state behind our back, as was the case in the past," Markovic said at the ceremony.

Foreign Minister Darmanovic submitted the small Balkan state's official accession papers. He hailed it as a historic day for the former Yugoslav republic, which only achieved independence from Serbia 11 years ago.

NATO Secretary-General Jens Stoltenberg called Montenegro's accession "good for the stability of the western Balkans. And it's good for international peace and security."

Moscow slams Montenegro's 'hostility'

Montenegro was officially extended an invitation to join the defense alliance in May, after its parliament had ratified the move a month before. The Kremlin slammed the decision, saying it would inflame regional tensions.

Russia's influence has diminished in Eastern Europe as more countries have joined NATO. That has had the additional effect of reducing the number of staging grounds that Russia could use for intervention in the Middle East.

"Russia reserves the right to take countermeasures on the basis of reciprocity amid the Montenegrin authorities' hostile stance," the Foreign Ministry announced in its statement.

Another factor casting a shadow over Montenegro's accession has been President Trump's attitude toward NATO. As president-elect he called the organization "obsolete."

After his inauguration Trump backed off the comments with the caveat that allies need to meet their financial pledges for military spending.